The present invention relates to an equalizing system in a receiver of a digital communication system, and, more particularly, to an adaptive equalizing method and apparatus for recovering a received digital signal more quickly and reliably.
In recent times, there has been several developments in modem technology for transmitting a full digital signal such as high definition television signal and for recovering the transmitted signal. In a modem, the signal transmitted from a transmitter includes noise due to multiplexed paths and is subject to interference and obstacles from other channels.
In high-definition television systems, however, channel errors are highly undesirable as data having a very high compression ratio through a frequency band of 6 MHz is transmitted. As a result, in high-definition television systems, there should be no errors in the transmission data even when there is severe channel distortion. In addition, high-definition systems require reliable modulation and demodulation and error correction to obtain digital data having a high compression ratio.
At present, according to such requirements, the receiver in a digital communication system performs an equalizing process on a received signal in which noise is included and of which the frequency characteristic is altered. Algorithms used in this so called adaptive equalization includes constant modulus algorithm (CMA), a stop-and-go algorithm (SGA) and a decision-directed algorithm (DDA), as examples. The CMA is discussed in "Self-recovering Equalizer and Carrier Tracking in Two Dimensional Data Communication System" by D. N. Godard (IEEE Transactions on Communication, Volume COM-28, No. 11, pp 1867-1875, November 1980). The SGA is also discussed in "Blind Equalization and Carrier Recovery using A Stop-and-Go Decision-directed Algorithm" by G. Picci and G. Prat (IEEE Transactions on Communication Volume COM-26, No. 9, September 1987).
The receiver performs a carrier recovery process for compensating for any phase error in the received signal. The receiver also compensates for phase offset, frequency offset, and phase jitter, etc., which cause mismatch between the carrier phase of the transmitted signal from the transmitter and the carrier phase of the received signal in the receiver, when recovering the carrier.
European Patent Laid-open Publication No. 0,524,559 A2 by Paik et al., published on Jan. 27, 1993, discloses a carrier phase recovery technique which is particularly useful for the recovery of multi-level amplitude modulation data, such as quadrature amplitude modulation. In this reference, a blind equalizing algorithm is used in a carrier recovery loop, in which an equalizing coefficient is initialized using the CMA. Then, if the phase error of the equalized signal matches a predetermined threshold value, the equalizing coefficient is initialized using the DDA.
When the DDA is used at the state of the initialization of the equalizing coefficient, the algorithm conversion between the CMA and the DDA is excessively sensitive to a degree of convergency. Accordingly, it is preferable to use the SGA instead of the DDA.